Definition of 無くてはいけない
-
(exp) cannot do without something; indispensable; absolutely necessary
君はもう大人なんだから、もっと分別がなくてはいけない。
Now that you are grown-up, you ought to know better. -
(after negative base of verb) have to do
すぐに手紙をかかなきゃいけないよ。
You've got to write to him quickly.
